Re: Amargasaurus...

["Steve Brusatte" <dinoland@lycos.com>]

On Wed, 25 Apr 2001 23:14:41  
 Marcel Bertolucci wrote:
>
>    If no one has anything else to say about _Carnotaurus sastrei_, since 
> we've had a very informative and helpful way to, at least, try to understand 
> this weird dinosaur, I'd like to talk about another weird one, of the same 
> country: _Amargasaurus cazaui_.
>    What are the most accepted theries to explain his vertebrae's long 
> vertical bifurcation?
>    I've seen one that thought these 'double spinal prolongments' were spikes, 
> to deffend him from predators. I don't think that may be possible, because I 
> see them very thin and dellicate to be used this way.

I am not usually one who generalizes, but I think that many of the bizarre 
structures that we see in dinosaurs and other vertebrate fossils are nothing 
more than sexual display characteristics, many of which are dimorphic.  

I'm not quite sure about Amargasaurus, as I've only seen photos and haven't 
really read much about it.  However, I think the vertebral structures you are 
referring to may be dimorphic.  

Do you really think that Amargasaurus would need spikes like these to defend 
itself from predators?  Based on my readings and studies and discussions, I 
believe that the large size of Amargasaurus would be a more than adequate 
defense.  Of course, it was up against some large predators.  

What would be very interesting to see is if juvenile specimens of Amargasaurus 
also possess these structures.  Have any juveniles been found?  If the 
juveniles lack them, I would definitely tend to say that they are dimorphic.  I 
am a big "believer" in Scott Sampson's work on ceratopsians.  He concluded that 
the bizarre and exorbiant crests, frills, and horns developed when the animal 
reached adulthood.  He reasoned that this signals dimorphism, because only 
animals that have reached sexual maturity would need structures for sexual 
display.  In addition, the juveniles would be most vulnerable to predators, so 
they would be the ones who would need these structures if they were truly for 
protection.  Of course, ceratopsian herds could have evolved a way to protect 
the youngsters from predation, making the point of having these defense 
structures worthless and moot in juveniles.  This has been discussed on the 
list before (look in the archives).  However, I do think that, for t!
he 
most part, ceratopsian crests and horns were used more for sexual reasons than 
defense.  
The same may be true for bizarre structures seen in other dinosaurs, including 
stegosaurids and ankylosaurs.  Unfortunately, sexual dimorphism in these 
dinosaur types, along with sauropods, is poorly understood.  

Steve

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